Hydrogen is a very important material in the chemical industry. For example, it is used to produce ammonia and methanol. Also, it is an essential material for hydrogenation in which unsaturated compounds are converted into saturated ones and also for hydrotreating processes, including hydrogen addition, desulfurization, denitrogenation and demetallization. Another example for the use of hydrogen is contact hydrogenation of carbon dioxide in which carbon dioxide, which causes global waning, is reclaimed, immobilized and reused. In addition, hydrogen is viewed as a pollution-free, clear energy source substituting for existing fossil fuels.
Conventional techniques for obtaining hydrogen include extraction from fossil fuels, such as naphtha, modification of natural gas, reaction of vapor with iron at a high temperature, reaction of water with alkaline metal, electrolysis of water, etc.
However, these techniques are economically unfavorable because immense heat or electric energy is required. Regarding modification of fossil fuels, the conventional techniques have another disadvantage of generating a large quantity of by-products, such as carbon dioxide. In case of electrolysis, problems, such as electrode lifetime and generation of by-products, need to be solved to purify hydrogen more easily. Thus, the cost of facilities for hydrogen production is economically unfavorable due to the noted problems.
Hydrogen gas readily escapes from the earth's gravity because it is of low specific gravity. Most of X exists in water or in inorganic forms. For these reasons, only a small quantity of hydrogen exists in the atmosphere. It is also very difficult and economically unfavorable to purify hydrogen existing in inorganic forms. The development of techniques to obtain high purity hydrogen efficiently from water is very important and urgently needed to exploit substitute energy sources.
Recently, hydrogen producing techniques have been developed in which a photocatalyst is used to decompose water into hydrogen and oxygen. However, little has been published in prior art relating to photocatalysts for producing hydrogen. Representative examples are: Japanese Pat. Laid-Open Publication Nos. Sho 62-191045 and Sho 63-107815.
Japanese Pat. Laid-Open Publication No. Sho 62-191045 relates to generating hydrogen from an aqueous Na.sub.2 S solution in the presence of a rare-earth element compound by a photolysis reaction The rarer element compound has an advantage of exhibiting optical catalytic activity in the range of visible light.
Japanese Pat. Laid-Open Publication No. Sho 63-107815 concerns a photolysis reaction in which a composite oxide of niobium and alkaline earth metal is used as a photocatalyst to generate hydrogen from a methanol solution in water. This photocatalyst likewise has an advantage of being active in the range of visible light.
However, both of these prior art methods are disadvantageous because the amount of hydrogen generated by them is as little as 10 ml/0.5 g hr.
Korean Pat. Appl'n. No. 95-7721, No. 95-30416, and No. 96-44214 solve the above problems to some degree by suggesting a photocatalyst represented by the following formula I: EQU Cs(a)/K.sub.4 Nb.sub.6 O.sub.7 I
This technique has little effect on the environment and generates hydrogen at room temperature. However, the oxygen-containing organic compounds, which act as hydrogen-generating promoters, make it impossible to reuse required reactants.
Korean Pat. Appl'n No.95-30416 suggests a photocatalyst represented by the following formula II EQU Cs(a)H(c)/S(b) II
This technique has little affect on the environment and generates hydrogen without an oxygen-containing organic compound as a hydrogen-generating promoter at room temperature, but encounters a problem with the lifetime and the stability of the photocatalyst.
For example, when an alkali metal, such as cesium is impregnated into a photocarrier, the amount of generated hydrogen is increased outstandingly but the stability of the catalyst is decreased.
In addition, Korean Pat. Appl'n No. 96544214 suggests a photocatalyst represented by the following formula III EQU Pt(a)/Zn[M(b)]S III
wherein "a" represents % by weight of Pt in the photocatalyst, ranging from 0.1 to 3.5; "M" represents a promoter selected from a group consisting of Co, Fe, Ni and P; and "b" represents mole % of M.
Similarly, this technique also has little affect on the environment. This compound shows not only the optical activity of photocatalyst in some degree but also the preparation is relatively simple and the stability of photocatalyst is superior. The lifetime of said compound is longer which depends on electron donors and reducing agents and the amount of generated hydrogen is larger than that of prior arts.
When doping with Pt instead of Cs the stability of the catalyst is improved but the choice for a promoter is less, and the amount of generated hydrogen is too little. In addition, there are some problems in the preparation of said photocatalyst. It needs sintering and rewashing twice followed by etching with an acid after primary sintering.